1. Field of the Invention
The present invention relates to a color television receiver, or more in particular to a beam-indexing color television receiver with an improved color reproducibility.
A beam-indexing color television receiver comprises a picture tube including a phosphor screen having phosphor stripes illuminated in red (R), green (G) and blue (B) by irradiation thereon of an electron beam and index phosphor stripes aligned in a predetermined position relative to the alignment of the phosphor stripes for emitting invisible light such as ultraviolet rays by irradiation thereon of an electron beam. An index signal obtained when an electron beam scans the index phosphor stripes is used to detect the electron beam scanning position, and on the basis of the resulting data the amount of electron beams is modulated by time so that the desired ones of the R, G and B phosphors are illuminated to the desired brightness, thus reproducing a color image of normal hue as well known.
The beam-indexing color television receiver is generally classified into the two types mentioned below according to the relative arrangement of sets (hereinafter referred to as triplets) of R, G and B phosphor stripes and the index phosphor stripes.
One of them, which is called the l/m type, is so configured that one index phosphor stripe is assigned to every m (integer) triplets. In the other type which is called the n/m type, n index phosphor stripes (m and n being integers which have no common measure) are assigned to every m triplets.
2. Description of the Prior Art
The n/m type has the advantage that substantially the same index signal is always obtained regardless of the hue reproduced on the screen of the receiver thereby to attain a uniform color reproducibility for each color, but the disadvantage thereof is that a start synchronizing means is required, resulting in various incidental shortcomings as well known. Specifically, the shortcomings include the facts that a start synchronizing index phosphor stripe is required, thereby reducing the effective screen size proportionally, that the configuration of the start sync signal processing circuit is complicated, that in a case of malfunction of the start sync signal processing circuit by noises mixing therein or other causes, the hue of the whole of a scanning line is completely disturbed, often leading to an abnormal image on the screen, and that in a case where a signal of low brightness level is included in a scanning line and the index signal is not obtained for that particular part, the hue of the reproduced color is disturbed for the remaining part of the scanning line. In order to solve this problem in the prior art, a beam current of 1 to 2 .mu.A, that is, a minimum required bias current is applied to the beam-indexing picture tube at all times, thereby stably detecting the index signal over the whole effective screen area. This method, however, gives rise to another disadvantage that as the bias current is applied to the beam-indexing picture tube all the time, the black level of the reproduced image (for example, the uncolored black part of hair) will not be reduced sufficiently, so that neither a sufficient contrast nor a sufficient color saturation is attained.
The greatest advantage of the l/m type, on the other hand, is that the start sync means is not required as well known. Although this eliminates the shortcoming of the n/m type, there are such disadvantages that the index signal is not produced at the time of reproduction of a specific color and that the index signal obtained by electron beam scanning involves a different phase error in each of the different reproduced colors, as described in detail later. The last-mentioned disadvantage is a phenomenon that the hue of the reproduced image is slightly deviated from the hue information of the received signal.
First, the disadvantage that the index signal is not obtained at the time of a specific color reproduction will be described in detail. FIG. 1 shows a sectional view of a phosphor screen of a beam-indexing color television picture tube (hereinafter referred to as a picture tube) of the l/l type having an index phosphor stripe for each triplet. Reference numeral 1 denotes a panel glass, on which a red, a green and a blue phosphor stripe designated by R, G and B respectively are coated. The shadowed portion 2 denotes a guard-band disposed between adjacent phosphor stripes. Numeral 3 denotes a metal back, and numeral 4 an index phosphor stripe, which is located between the R and G phosphor stripes in the drawing under consideration. The beam current waveforms for reproducing red, green and blue colors by use of this picture tube are shown by solid lines in FIGS. 2(a), 2(b) and 2(c), respectively. In the figure, a beam current actually flows in the portion higher than the 0 level where the beam current is cut off. The portion of dashed lines under the 0 level shows that the waveform of the voltage for driving the picture tube is a sine wave. In FIG. 2, the shadowed parts show the portions in which an electron beam irradiates the index phosphor stripes 4 of FIG. 1 thus producing an index signal. In a case where red and green shown in FIGS. 2(a) and 2(b) are reproduced on the picture tube screen, an index signal is generated. In a case where the blue color shown in FIG. 2(c) is reproduced, however, no index signal is produced since the electron beam is cut off at the position of the index phosphor stripes 4 as seen from the figure. In a case of the picture tube having the phosphor screen shown in FIG. 1, the specific color for which no index signal is produced is blue. If the relative positions of the index phosphor stripes are different with respect to the triplet arrangement, however, the specific color for which no index signal is produced is naturally different. The case where blue is the specific color for which no index signal is produced will be described below. By the way, it is self-explanatory that in the case of the 1/3 type with one index phosphor stripe assigned to every three triplets, there is a specific color for which no index signal is produced.
When a color saturation is high and the peak current value is large such as in the case of a color bar signal, a large electron beam diameter is involved, with the result that even a slight deviation of focus adjustment will cause the index phosphor stripes to be irradiated by the beam for reproduction of a specific color, thus making the receiver operation unstable.